The present invention relates to a method for controlling the width of a glass ribbon manufactured through a float process and more particularly to a control method in which the rate of supplying a molten glass onto a molten metal in a bath is manipulated by regulating the height or opening of a tweel so as to reduce an error between a measured glass ribbon width and a predetermined standard width.
In a float process of flat glass production, the rate of supplying a molten glass fluctuates depending on operational conditions of a melting furnace to make molten glass, for example, a rate of charging the furnace with raw materials and regulating condition of a heavy-oil burner of the furnace. The inventor of the present invention has found that when the temperature of a molten glass increases bringing about a lowering of the viscosity of the molten glass and hence an increase of the rate of molten glass supply, the thickness of the glass ribbon is scarcely changed but remains within a dimensional range required by specifications of production while the width of the glass ribbon grows larger. That is, a fluctuation of the rate of molten glass supply has substantially no effect on the thickness of the glass ribbon but has a material effect only on the width of the glass ribbon.
Because a molten glass in general has a high viscosity, a glass ribbon on a molten metal does not exhibit a stable width when hot just after discharged onto the molten metal bath. Accordingly a detecting device to find a glass ribbon width by detecting positions of both side edges of the glass ribbon must be placed sufficiently distant from the tweel to the downstream and therefore there is a substantial time delay between a change of the tweel height and a responsive change of the glass ribbon width.
In such a process the value of the glass ribbon width tends to oscilate in a large amplitude alternately going over and under a standard value and therefore it has been very difficult to maintain satisfactorily the glass ribbon width uniform.
A fluctuation of the glass ribbon width causes an increase of area of the both side portions of the glass ribbon which must be trimmed off for final products, resulting in a decreased yield of flat glass products. In some cases the glass ribbon on the molten metal bath is held at the both side portions thereof by supporting rolls or the like and this causes a further increase of the trimmed off portions.